Field of the Invention
The present invention relates to a facimile system, or more in particular to a facsimile system of the analog type in which the white background of the original is skipped.
Characters or patterns including drawings on an original document which are to be transmitted by a facsimile system account for only 20% to 30% of the white background of the document. Most of the time required for transmission is consumed in the scanning of the white background. In high speed transmission, this naturally requires a higher scanning speed and hence a higher maximum image current frequency, thus making it difficult to use the telephone line of limited frequency band.
In order to overcome such a shortcoming, a special modulation system, such as the vestigal side band modulation and demodulation system (VSB) or the pulse code modulation system (PCM), has be suggested, in which the transmission time is reduced by lessening the frequency band of image current. Such modulation systems, however, enable the transmission speed to be increased only at most about twice in the same frequency band of the transmission circuit.
The International Telegraph and Telephone Consultative Committee (CCITT) has suggested a specification for the analog type facsimile apparatus (medium speed facsimile) which enables transmission and receipt of the information on an original of "A4" size in about three minutes. The systems based on that specification are called the group II (GII) systems and are used for general office purposes. It takes about three minutes for the group II (GII) systems to transmit any original. In other words, the same transmission time is required regardless of how much the white background represents of the area of the original.